A conventional angular speed sensor will be described below.
FIG. 11 is a circuit block diagram of the conventional angular speed sensor. In FIG. 11, the conventional angular speed sensor has oscillator 1 having driving electrode 2, monitor electrode 3, and detecting electrode 4, driving circuit 5, oscillation control circuit 6, detecting circuit 7, fault diagnosing circuit 8, and filter circuit 9. Driving circuit 5 applies a driving signal to driving electrode 2 to oscillate oscillator 1. Detecting circuit 7 detects an angular speed according to a detecting signal outputted from detecting electrode 4 due to an inertial force. Oscillation control circuit 6 controls an energization amount of the driving signal according to a monitor signal outputted from monitor electrode 3. Fault diagnosing circuit 8 inputs a pseudo angular speed signal to detecting circuit 7 for fault diagnosis.
FIG. 12 is an equivalent circuit diagram of filter circuit 9 included in detecting circuit 7. In filter circuit 9, an input signal is inputted to one input terminal 10a, the other input terminal 10b is grounded, and an output signal is outputted from output terminal 11. The input signal is inputted to an input portion (IN) via resistor 20 (R1). Resistor 12 (R2) and capacitor 13 (C1) are connected in parallel to input terminal 10a and output terminal 11. A filtered signal is outputted from an output portion (OUT) of filter circuit 9.
The detecting signal is filtered by filter circuit 9 of detecting circuit 7 and is outputted as a predetermined signal. An angular speed is detected according to the predetermined signal. The filter is a low-pass filter or a high-pass filter. A cutoff frequency of filter circuit 9 is about 10 Hz.
In the fault diagnosis of the inertial force sensor, as shown in FIG. 11, the pseudo angular speed signal for fault diagnosis is inputted to detecting circuit 7 by fault diagnosing circuit 8 at the time of start after power-on. When the angular speed is detected according to the pseudo angular speed signal, it can be judged that no faults have occurred.
As conventional document information on the invention of this application, Patent Document 1, for example, has been known. In the above conventional configuration, the detecting signal is filtered by filter circuit 9 of detecting circuit 7. As shown in FIG. 12, resistor 20 (R1), resistor 12 (R2), and capacitor 13 (C1) are included in filter circuit 9. These are typically delay elements. Here, a transfer function of filter circuit 9 is expressed by Equation 1, a phase characteristic thereof is expressed by Equation 2, and a delay time thereof is expressed by Equation 3.
                              T          ⁡                      (            ω            )                          =                                            -              R                        ⁢                                                  ⁢            2                                R            ⁢                                                  ⁢            1            ⁢                          (                              1                +                                                      jω                    ·                    C                                    ⁢                                                                          ⁢                                      1                    ·                    R                                    ⁢                                                                          ⁢                  2                                            )                                                          [                  Equation          ⁢                                          ⁢          1                ]                                          Φ          ⁡                      (            ω            )                          =                  arctan          ⁡                      (                                                            -                  ω                                ·                C                            ⁢                                                          ⁢                              1                ·                R                            ⁢                                                          ⁢              2                        )                                              [                  Equation          ⁢                                          ⁢          2                ]                                T        =                              δΦ            ⁡                          (              ω              )                                /          δω                                    [                  Equation          ⁢                                          ⁢          3                ]            
According to Equations 1 to 3, the delay time of filter circuit 9 is caused by the delay elements. When values of resistor 12 (R2) and capacitor 13 (C1) are large, time constant τ defined by Equation 4 is increased. Accordingly, the delay time becomes longer.τ=C1×R2  [Equation 4]
When the delay time becomes longer, as shown in FIG. 13, the output of the detecting signal is delayed with respect to pseudo angular speed signal 21 for fault diagnosis. Responsibility at the time of fault diagnosis is lowered. Accordingly, the diagnosis accuracy of fault diagnosis is lowered. To shorten the delay time, the values of resistor 12 (R2) and capacitor 13 (C1) as the delay elements should be smaller. However, when the values of resistor 12 (R2) and capacitor 13 (C1) as the delay elements are smaller, the cutoff frequency is increased, which affects angular speed detection accuracy. The above configuration has the problem that the accuracy of fault diagnosis cannot be improved by shortening the delay time while lowering the cutoff frequency.    [Patent Document 1] Unexamined Japanese Patent Publication No. 2002-267448